US2580983A - Corrosion inhibitors in absorption refrigeration system - Google Patents

Description

Jan. 1, 1952 N. E. WIDELL 2,580,933

CORROSION INHIBITORS IN ABSORPTION REFRIGERATION SYSTEM Filed Feb. 8. 1946 IN V EN TOR.

Patented Jan, 1, 1952 CORROSION INHIBITORS IN ABSORPTION REFRIGERATION SYSTEM Nils Erik Widell, Stockholm, Sweden, assignor to Aktiebolaget Eiektrolux, Stockholm, Sweden, a corporation of Sweden Application February 8, 1946, Serial No. 646,394 In Sweden January 22, 1945 15 Claims. (Cl. 62-179) This invention relates to absorption refrigerating apparatus and more particularly to such apparatus of the inert gas type wherein the absorption solution contains a corrosion protective substance. The main object of the invention is to increase the lifetime of the apparatus and to prolong the durability of the corrosion protection. The invention is of particular importance in such refrigerating apparatus wherein absorption solution from its normal circulating system through the boiler and absorber of the apparatus is brought into contact with working medium having a higher refrigerant concentration than that of the absorption solution.

The invention will be hereinafter more fully described with reference to the accompanying drawing showing diagrammatically an embodiment of the arrangement according to same and in this connection further characteristic features of the invention will be set forth.

The figure shows diagrammatically an absorption refrigerating apparatus, wherein water is used as an absorbent and ammonia as refrigerant. Referring to the figure reference numeral l denotes a tube acting as a flue, to which heat is continuously supplied from a heat source, not shown, for example an electric cartridge, 9. gas or petroleum burner. The boiler of the apparatus is designated by I I and its liquid circulating pump by I2. The boiler pipe II is extended upwardly and at the top connected to the air cooled condenser I3 of the apparatus. Further numeral I4 denotes a so-called pressure equalizing vessel of a kind known per se which through a drainage conduit I5 communicates with the evaporator of the apparatus designated by I6. In said conduit I5 the condenser pipe I3 opens, from which liquid refrigerant thus flows by gravity to the evaporator through the conduit I5. Further numeral Il designates the gas heat exchanger of the apparatus and I8 its air-cooled absorber. Numeral I9 indicates a vent conduit through which the pressure equalizing vessel I4 freely communicates with the gas circulating system of the apparatus. Said system is formed in a way known per se as is the liquid circulating system of the apparatus. The enriched solution in the absorber vessel 20 of the apparatus thus streams through its liquid heat exchanger 2| and the main pump l2 to the boiler I I and thence it returns through the heat exchanger 2i and a conduit 22 to the absorber I8.

In accordance with usual practice the apparatus is formed of iron pipes joined together by gas welding. Before filling the apparatus it is iii evacuated of all air, and as ammonia by itself has no influence upon iron in absence of oxygen, no corrosion should be feared in the internal iron walls. However, these walls formed of ferrous metal will nevertheless be affected by the ammonia solution by the forming of complicated iron compounds, by which the walls after a relatively short time would be loosened up-and lose their strength against the high pressure (about 20 atm.) prevailing in the apparatus. In order to avoid such corrosion an alkali salt of sodium chromate (NazCrOr) has previously been dissolved in the absorption solution. The chromatic concentration required in the absorption solution for counteracting the corrosion is extremely small and may be counted in fractions of per cent. However, the chromate is slowly consumed and therefore it has previously been necessary to utilize a concentration of chromate in the solution many times higher for the purpose of creating a chromatic reserve, covering several years consumption. Salts dissolved in the absorption solution will, however, influence the pumping in the thermosiphon pump I2 in a unfavourable direction and therefore until now it has been necessary to limit the concentration of chromate in the solution to a concentration corresponding to about 2% of 0103.

In different ways, however, it has been attempted toform an additional reserve of alkali chromate by enclosing in the absorber vessel a storage of solid sodium chromate in a container with a limited communication with the surrounding solution, whereby a gradual dissolving of the solid chromate has been attained. It has not been possible to make the dissolving so uniform that the concentration of the solution with security can be kept below the values wanted. The velocity of dissolving has also appeared to be dependent upon a plurality of uncontrollable factors, among other things the operating temperature which very essentially varies with the climatic concentration under which the apparatus is operated. According to this invention a reserve of chromate of an arbitrary greatness may be arranged in the apparatus. The invention is substantially characterized thereby that the absorption solution of the apparatus is saturated with a salt of the chromatic acid, having a solubility in said solution not exceeding 2%. Further the solubility of the present chromate may be as much as possible independent of the temperature, though certain variations with the temperature may be permitted. A particularly suitable chromate is strontium chromate (SrCr04). The solu- 3 bility of strontium chromate in the absorption solution is so small that the saturated solution only contains 0.01 to 0.2% of CrOa.

Apparently this small quantity of chromate which originally, when filling the apparatus, is dissolved in the absorption solution will be comparatively rapidly consumed. It is therefore generally necessary to enclose a reserve of solid strontium chromate in the apparatus. According to the invention therefore a container 50 is arranged in the absorber vessel 20 or possibly in another vessel interconnected with the liquid circulating system of the apparatus. The reservoir 50 may suitably include a short piece of pipe, both openings of which are covered with a fine-meshed iron net and in which a pressed tablet of strontium chromate is inserted. The container may suitably be attached to the wall of the vessel by a strip or similar member which is attached to the wall of the vessel and to the wall of the container 5|! by welding. The support is denoted by El. When the liquid contents of the absorber vessel is continuously renewed, the absorption solution will automatically become saturated gradually as its contents of chromate decreases during its passage through the absorber vessel 20. The pressing of the solid strontium chromate into a tablet possesses the advantage that no solid particles by mechanical influence of the streaming solution will follow the liquid through the narrow passages of the apparatus. In order to improve the strength of the tablet small amounts of sodium hydroxide may be mixed with the chromate when pressing said tablet.

As a ready mentioned, the invention is of particular impo tance in such absorption refrigerating apparatus in which the absorption solution is now and then brought into contact with liquid refrigerant, the concentration of which thus attains high values. This is the case in the absorpto the main pump l2 the auxiliary pump 23 is d not immediately heat conductively connected with the flue ill but instead arranged in heat conductive relation with a hermetically closed heat transfer system which is welded to the flue in along a welded seam 42. The system contains a heat transfer meduim with a high boiling point, possibly in addition to an auxiliary medium with -a lower boiling point. The transfer medium is vaporized in the boiling part ll of the system and condenses to liquid in a container where the condensate is accumulated. Some part of the vapours, however, give oil their heat contents to the pump pipe 23. After a certain period the system is boiled dry, and all the condensate remains storaged in the container 30. This container is connected by a flexible member 3| with the system, thus permitting it to be raised for tilting the liquid contents. In this way a method of controlling the heat transfer system is attained, thus causing the pump 23 to operate whenever desired and after a certain period automatically to stop working. The solution raised by the auxiliary pump 23 streams in the way already described into the evaporator where 4 entering from the condenser 13, having an em.- monia concentration from 80 to 100%. The supply of condensate from the condenser I3 takes place continuously. This supply of absorption solution to the evaporator 16 has for its object, when required, to effect melting of ice formed thereon. It has appeared that a solution of sodium chromate of 1 to 2 per cent cannot be mixed with ammonia having the concentration existing in the evaporator without most of the salt bein precipitated into solidform. The salt precipitated would collect in the evaporator and would very soon effect a blocking action and at the same time such precipitated salt for the future would be prevented from taking part of the corrosionprotection. A solution of strontium chromate remains, even if it is saturated, approximately unaffected by an increase of the ammonia concentration and the small amounts of chromate which possibly may be precipitated will be replaced from the reserve provided in the absorber vessel.

Though the strontium chromate is very little so uble in the absorption solution, that is, in the ammonia water mixture, the concentration of protecting substance in this chemical compound is sufilclent to effect a satisfying corrosion protection. Other chromates, such as lead chromate, neutral or basic (PbCr04 or PbCr04, PbO) are practically unsoluble in the ammonia water solution. Accordingly such compounds alone and by themselves have no corrosion protective effect. According to the invention, however, the active substance of the lead chromate-may be freed from the lead when in the absorption solution an activating substance is dissolved, such as sodium hydroxide (NaOH) or any other substance which when disintegrated forms sodium hydroxide. According to another phase of the invention, sodium hydroxide, for examp e 2%, is dissolved in the absorption so ution. This solu. t on will then effect a disintegration of the lead chromate. substantially according to the following form:

The sodium chromate formed thereby will be di solved by the absorption solution and distributed into all parts of the circulation system of the absorption solution. The lead compound NaHPbO-z is also soluble in the absorption solution, and therefore will not eilect any disturbances. According to the small concentration of the sodium hydroxide in the solution, the quantity of sodium chromate formed and dissolved therein in accordance with the foregoing eouation will be small and will remain at a substantlally constant value as the reaction proceeds until all lead chromate has been dissolved.

In some cases the low concentration of the chromate in the absorption solution will be insufficient for effecting a satisfactory protection against corrosion. According to a further phase of the invention, however, a further corrosion protective substance, such as the nitrites of alkali metals, for example, may be added to the absorption solution and dissolved therein until a concentraticn of about 4% is attained. Such nitrites of the alkali metals may include ammonium, potassium or sodium nitrite. Preferably sodium nitrite (NaNOz) is used. Though this chemical compound by itself is known as being very unsta ble investigations have shown that the duration of the protective eilfect is suilicient for the above it, however, is mixed with refrigerant condensate 7i} purpose and may be very much increased when a chromate, also a very diluted solution of chromates, is used in combination with the nitrites. The concentration of chromates suillcient for effecting such an increase of the duration of diluted nitrite solutions corresponds to about 0.02 to 0.2% CIOa. It is evident that nitrite solutions containing 0.02 to 4% NaNO-i and saturated with strontium chromate or lead chromate is very suitable as absorption solutions in refrigerating apparatus of the above described type, due to the fact that sodium nitrite when acting as a corrosion protective substance will be disintegrated forming thereby gaseous products and sodium hydroxide. This sodium hydroxide will dissolve lead chromate as described above, and accordingly activate said compound which in itself is practically insoluble and inactive. The sodium hydroxide and the sodium nitrite respectively accordingly act as activators for the lead chromate. The sodium chromate dissolved thereby prevents the rapid disintegration of the nitrites.

It is evident that the invention is of greatest importance in connection with hermetically closed refrigerating apparatus of the type described in connection with the figure. The lifetime of such apparatus is substantially determined by the duration of the corrosion protection. According to the invention it is now possible to enclose in such a paratus a corrosion protective substance soluble in the absorbent and useable in high concentration, and a second corrosion protective substance acting as conservator for said first corrosion protective substance and a reserve of solid corrosion protective substance, from which an active part is dissolved by means of the disintegration products of the first-mentioned corrosion protective substance. Due to this fact the corrosion protection may be made effective for an extremely long duration.

I claim:

1. In the art of refrigeration employing an hermetically sealed absorption refrigerating system formed of ferrous metal and having an absorption solution circuit in which aqueous ammonia solution is circulated through and between a place at which ammonia is expelled from solution and a place at which ammonia is absorbed into solution, the improvement which comprises maintaining in the solution a supply of solid lead chromate, and providing in the solution an alkali nitrite.

2..The improvement as set forth in claim 1 in which sodium nitrite is provided in an amount to obtain a concentration of from 0.02 to 4.0 per cent.

3. In the art of refrigeration employing an hermetically sealed absorption refrigerating system formed of ferrous metal and having an absorption circuit in which an aqueous ammonia solution is circulated through and between a place at which ammonia is expelled from solution and a place at which ammonia is absorbed into solution, the improvement which comprises maintaining in the solution a supply of solid lead chromate, and making available in the solution an alkaline hydroxide.

4. The improvement as set forth in claim 3 in which about 2 per cent sodium hydroxide is added to the solution.

5. In the art of refrigeration employing an hermetically sealed absorption refrigerating system formed of ferrous metal and containing ammonia, water and hydrogen and comprising an evaporator and a number, of parts interconnected to provide a circuit for circulation of aqueous ammonia solution and in which such solution is supplied at will from said circuit to the evaporator to effect defrosting of the latter when desired, the improvement which comprises adding to the solution an alkaline nitrite, and maintaining in said circuit a supply of a chromate salt of lead which is in solid form.

6. The improvement as set forth in claim 5 in which sodium nitrite is added to the solution.

7. In the art of refrigeration employing an absorption refrigeration system formed of ferrous metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises maintaining in the aqueous ammonia solution in physical contact therewith a supply of a chromate salt of lead soluble in the solution only with difllculty and having bound therein a chromate radical inactive due to the insolubility of said lead salt, and adding to the solution another substance which upon dissolution or disintegration will provide an alkaline hydroxide to effect liberation of said chromate radical by chemical action.

8. In the art of refrigeration employing an absorption refrigeration system formed of ferrous metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises maintaining in the aqueous ammonia solution in physical contact therewith a supply of a chromate salt of lead soluble in the liquid only with difliculty and having bound therein an inactive chromate radical capable of forming a, readily soluble corrosion inhibiting chromate salt upon liberation from said lead salt, and providing in the solution an alkali hydroxide which will eifect liberation of said inactive chromate radical from said lead salt and promote formation of said corrosion inhibiting chromate salt.

9. In the art of refrigeration employing an absorption refrigeration system formed of ferrous metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises maintaining in the aqueous ammonia solution in physical contact therewith a supply of a chromate salt of lead soluble in the solution only with difliculty and having bound therein a corrosion resisting chromate radical inactive due to the insolubility of said lead salt, and adding to the solution an alkali nitrate eifective to inhibit corrosive action which upon disintegration thereof will liberate said corrosion resisting chromate radical by chemical action.

10. In the art of refrigeration employing an absorption refrigeration system formed of ferrous metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises maintaining in the aqueous ammonia solution in physical contact therewith a supply of chromate salt of lead soluble in the solution only with difficulty and having chemically bound therein an inactive chromate radical capable of forming a readily soluble corrosion inhibiting chromate salt upon liberation from the said lead salt, adding to the solution an alkali nitrate which upon dissociation is effective to liberate said inactive chromate radical from the said lead salt to produce said readily soluble corrosion inhibiting chromate salt, and utilizing said readily soluble corrosion inhibiting chromate salt to retard and slow down rate of dissociation of said alkali nitrate.

11. In the art of refrigeration employing an absorption refrigeration system formed of fermucosa:

rolls metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises maintaining in the aqueous ammonia solution in physical contact therewith a supply of solid lead chromate compound (PbCrO4 or PbCrO Pb),said compound being soluble in the solution only with difficulty and thereby rendering the chromate inactive as a corrosion inhibiting agent, and liberating the chromate from said compound and forming a corrosion inhibiting chromate compound soluble in the solution by adding a substance selected from a group consisting of an alkali nitrite or an alkali hydroxide;

12. In the art of refrigeration employing an absorption refrigeration system formed of ferrous metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises maintaining in the aqueous ammonia solution a solid substance comprising a chromate salt of lead, and adding an alkali nitrite to the solution.

13. In the art of refrigeration employing an to provide a circuit for active circulation of aqueous ammonia solution having a corrosion inhibiting chromate salt soluble therein, said parts including a, boiler or generator and an absorber comprising a looped coil and vessel connected thereto, means to provide said soluble corrosion inhibiting chromate salt including a tablet in solid form comprising a substance selected from a group consisting of strontium chromate (SrCrOn, neutral lead chromate (PbCr04) or basic lead'chromate (PbCrOr, PhD), and means comprising a receptacle for holding said tablet in said vessel in intimate contact with the aqueous ammonia solution therein at a region removed from the walls defining said vessel.

15. In the art of refrigeration employing an absorption refrigeration system formed of ferrous metal and utilizing ammonia as refrigerant and water as an absorbent, that improvement which comprises providing in the system a soluble alkali nitrite and a solid substance selected from a group consisting of strontium chromate (SrCrOd, neutral lead chromate (PbCrOO or basic lead chromate (PbCrO4, PbO) NILS ERIK WIDELL.

REFERENCES CITED The following references are of record in the iiie of this patent:

UNITED STATES PATENTS Number Name Date 1,960,040 Widell May 22, 1934 2,210,611 Widell Aug. 6, 1940 2,246,665 Buflington June 24, 1941 2,402,413 Kogel June 1, 1946 2,457,334 Widell --Dec. 28, 1946 FOREIGN PATENTS Number Country Date 117,816 Australia Dec. 1943

Claims (1)

1. IN THE ART OF REFRIGERATION EMPLOYING AN HERMETICALLY SEALED ABSORPTION REFRIGERATING SYSTEM FORMED OF FERROUS METAL AND HAVING AN ABSORPTION SOLUTION CIRCUIT IN WHICH AQUEOUS AMMONIA SOLUTION IS CIRCULATED THROUGH AND BETWEEN A PLACE AT WHICH AMMONIA IS EXPELLED FROM SOLUTION AND A PLACE AT WHICH AMMONIA IS ABSORBED INTO SOLUTION, THE IMPROVEMENT WHICH COMPRISES MAINTAINING IN THE SOLUTION A SUPPLY OF SOLID LEAD CHROMATE, AND PROVIDING IN THE SOLUTION AN ALKALI NITRITE.

Images